1. Field of the Invention
This invention relates to thermometers and in particular to disposable, or throwaway, thermometers.
2. Description of the Prior Art
The conventional clinical thermometer comprises a glass tube defining at one end a reservoir bulb and having an axial capillary bore which is conventionally closed at the end opposite the reservoir bulb. The indicator liquid is sealed within the glass element and a suitable scale is provided on the outer surface thereof for indicating a temperature corresponding to the level of the miniscus of the indicator liquid expanded into the capillary bore. To provide an accurate reading, the capillary bore is effectively evacuated to provide a preselected vacuum condition which is effectively maintained over the life of the thermometer by the gas impervious characteristics of the glass.
A serious problem, however, has arisen relative to the use of such thermometers in clinical applications relative to the possibility of cross infection which occurs when the thermometer is reused. The cost of such glass thermometers is such that they cannot be considered as throwaway, one-time use thermometers, and thus there has been a strong need for a low cost substitute for such glass thermometers in applications, such as clinical applications.
One apparently obvious solution to this vexatious problem attempted by numerous investigators in this field has been to use a synthetic resin substitute for the glass material. As is well known, synthetic resin material may be molded at low cost in large quantities and, thus, the synthetic resins would appear at first blush to offer an almost obvious solution to this problem. Thus, it has been that a large number of different types of synthetic resin thermometers have been developed. Plastic molding techniques are sufficiently accurate to provide extremely accurate capillary bore dimensions and prototypes of such synthetic resin thermometers have, when produced, shown original conformation to the stringent requirements of such clinical thermometer applications.
However, such synthetic resin thermometers have proven completely unacceptable as a substitute for such glass thermometers. More specifically, it has been found that while such synthetic resin thermometers may be formed to have the required accuracy for such clinical use, after a relatively short period of time, such as several weeks, the thermometers lose their accuracy and are completely unacceptable for such clinical use. It has been found that the cause of this loss of accuracy resides in the inability of known synthetic resins to prevent gaseous diffusion therethrough so that the original vacuum condition within the thermometer cannot be maintained over a period of time.
One novel solution to this problem has been recently developed. Thus, it has been proposed to eliminate the vacuum condition within the capillary bore by providing a small access opening at the distal end of the capillary bore through the plastic element so that the indicator liquid within the thermometer is always at atmospheric pressure. Thus, it is immaterial whether or not diffusion occurs through the plastic material. This novel approach to the problem is exemplified in the following U.S. Pat. Nos. 3,739,642 and 3,938,388 of Josef F. Klingler.
However, the open-ended thermometer solution has one defect in that the indicator liquid is exposed to atmospheric contaminants at all times. Absorption of such contaminants, such as water vapor, contaminant gases, etc., may alter the effective volume and other characteristics of the indicator liquid so that some effect on the accuracy of such thermometers may occur from such causes in normal storage and use. Thus, while the open-ended thermometer may provide one solution to the disposable thermometer problem, there still remains a strong need for a simple sealed thermometer which, in substantially all respects, is similar to the conventional glass thermometer but yet is formed of a low cost synthetic resin material permitting the thermometer to be a one-time use, throwaway thermometer.